Q.Mycorrhiza does not help the host plant in:

Mycorrhiza is a symbiotic association between fungal hyphae and plant roots that benefits the host in several well-documented ways. The fungal partner extends far into the soil, dramatically increasing the surface area for absorption. This network is particularly efficient at taking up phosphorus, a relatively immobile nutrient that plants often struggle to access — so (A) is a genuine benefit.

The extensive hyphal network also improves water uptake, which directly enhances the plant's ability to withstand drought conditions — (B) is correct as well. Additionally, mycorrhizal fungi can form a protective sheath around roots and produce compounds that suppress soil-borne pathogens, thereby increasing resistance to root diseases — (C) is also a real advantage. …

Mycorrhizae represent one of nature's most elegant partnerships. The term itself comes from Greek—"myco" for fungus and "rhiza" for root—and describes the symbiotic association between certain fungi and the roots of most vascular plants. This relationship is ancient, dating back hundreds of millions of years, and has been crucial in allowing plants to colonize land.

The fungal partner in this association extends far beyond the root system through an extensive network of thread-like hyphae. These hyphae act as extensions of the root system itself, dramatically increasing the surface area available for absorption. The plant, in return, supplies the fungus with carbohydrates produced through photosynthesis—a classic mutualistic exchange where both partners benefit.

What mycorrhizae actually do for plants

The benefits mycorrhizae confer to their host plants are substantial and well-documented:

Nutrient uptake enhancement is perhaps the most celebrated function. Phosphorus, despite being abundant in many soils, exists largely in forms plants cannot directly absorb. The fungal hyphae secrete enzymes and acids that solubilize bound phosphorus, making it available to the plant. The vastly increased absorptive surface area also helps capture phosphorus from a much larger soil volume than roots alone could access. This is why option (A) is correct—mycorrhizae are exceptional at enhancing phosphorus uptake.

Drought tolerance improves significantly in mycorrhizal plants. The hyphal network explores soil pores too small for root hairs to penetrate, accessing water reserves unavailable to non-mycorrhizal plants. The fungi also help maintain soil structure, improving water retention. During dry periods, this extended reach can mean the difference between survival and death, making option (B) a genuine benefit.

Protection against root pathogens occurs through multiple mechanisms. Mycorrhizal fungi physically occupy root space that might otherwise be colonized by pathogens. They also trigger the plant's own defense responses and may produce antibiotics that suppress harmful organisms. Some mycorrhizae even alter root exudates in ways that make the root environment less hospitable to disease-causing organisms. Option (C) therefore represents a real advantage. …

A different angle: sort the four options by WHERE in the plant the effect happens, not by what mycorrhiza generally does.

Mycorrhizal fungi live and act at the root–soil interface. Ask, for each option, whether the benefit is delivered through that root-zone channel:

• (A) Phosphorus uptake — happens through root-associated hyphae. In the root zone. ✓
• (B) Drought tolerance — comes from the hyphal network reaching soil water. In the root zone. ✓
• (C) Root pathogen resistance — literally at the root surface. In the root zone. ✓ …